1. Technical Field
The present disclosure relates to a mounting method and apparatus for mounting conductive balls on a substrate. More particularly, the present disclosure relates to a mounting method and apparatus for mounting one conductive ball on each of a plurality of pads for external connection on a substrate (e.g., a wiring substrate, a package substrate, or a wafer).
2. Background Art
Upon forming solder bumps for connection with other electronic components on pads formed on a substrate such as a wiring substrate or a wafer, in the related art, solder paste is transferred on the pads by printing, and is formed into semispherical bumps by heating. In this method, as the pitch between the pads becomes narrower with an increase in density of electronic components, a short circuit caused by bridge between adjacent pads becomes remarkable. In forming bumps with a printing method, it is considered that the bump pitch has a limitation of about 200 μm.
With regard to formation of bumps at a narrower pitch, there is known a method utilizing solder balls each previously formed into a sphere. According to this method, for example, JP-A-11-297886 discloses the following steps. Firstly, a metal mask having through holes each receiving one solder ball at positions corresponding to each of the pads on a substrate is aligned with and overlapped on the substrate. Then, a large number of solder balls put on the mask are dropped into the through holes one by one. Then, the solder balls are allowed to adhere to the flux coated on the pads in advance. Then, excess solder balls on the mask are removed by a squeegee. Lastly, the mask is removed. As a result, solder balls are mounted on the substrate.
Meanwhile, the inventors of the present application developed a method for mounting solder balls on a substrate in the following manner, and filed a patent application as to the method (see U.S. patent application Ser. No. 12/042,916) According to this method, firstly, a metal mask having through holes each receiving one solder ball at positions corresponding to each of the pads is overlapped on a substrate. Then, with the solder ball mounting side of the substrate facing downward, a container in which the solder balls are contained is placed under the substrate. Then, by blowing air through the holes formed in the bottom of the container or by vibrating the container, the balls are moved up and allowed to pass through the respective through holes of the mask, and then are allowed to adhere to the flux coated on the pads in advance. Lastly, the mask is removed.
When the foregoing method in which balls are dropped one by one into respective through holes of the metal mask is used to mount conductive balls such as solder balls for forming bumps on a substrate with pads at a pitch as narrow as 200 μm or less, a step of arranging the balls on the pads by dropping and a step of removing the balls remaining on the mask are required to be performed in separate steps. This unfavorably results in a longer time required for the mounting step. Further, when the balls remaining on the mask are removed, the ball surface is likely to be damaged due to friction with the mask. Then, the balls become more likely to be oxidized, which unfavorably hinders reuse of the collected balls.
In using the method in which balls are moved up by blowing air, the balls are blown outside the container upon strengthening the power of blown air. Therefore, it is difficult to allow the balls to move up at a high density in one step. For this reason, in this case, an improvement of ball mounting rate is demanded.
In using the method in which balls are moved up with vibration, the balls are moved up uniformly. However, upon mounting the balls on a high density pad pattern, the balls cannot be supplied to the prescribed respective pads well because the moved up balls collide with each other. Furthermore, when the amount of the balls in the container is increased, the balls cease to move up.
Thus, in the related-art method for mounting conductive balls such as solder balls on a substrate with pads at a narrow pitch, it is very hard to simultaneously satisfy shortening of the mounting time, improvement of mounting rate, and simple reuse of the collected balls that are not mounted.